Recently, the worldwide energy crisis is becoming even more serious. Thus fully utilizing energy resource and decrease the energy waste becomes more and more urgent. In the consumer electronic products, the desktop computers and the workstation computers occupy a quite large percentage, and the low efficiency of the switched-mode power supply of these products has brought a great deal of energy waste. For that matter, Energy Star increases the forced efficiency standard to more than 80% when switched-mode power supply for the desktop computers and the workstation computers load from 20% to 100% since Jul. 20, 2007. Therefore, the design of a switched-mode power supply with high efficiency in a wide load range is desired.
The switched-mode power supply of the desktop computers and the workstation computers usually has a multi-outputs structure, and the output voltages of which include +12 V, +5 V, and +3.3 V in general. FIG. 1 shows a circuit diagram of a DC/DC converter stage of a desktop computer in the prior art. In FIG. 1, the DC/DC converter stage 1 includes an inverter, a transformer T having a primary winding Np coupled to the inverter, a first secondary winding Ns1 and a second secondary winding Ns2, a rectifier coupled to the first secondary winding Ns1, a first filter (including a first inductor L1 and a first capacitor C1) and a second filter (including a second inductor L2 and a second capacitor C2) both coupled to the rectifier and respectively employed to generate output voltages of +12 V and +5 V, a post regulator coupled to a node of the first secondary winding Ns1 and the second secondary winding Ns2, and generating an output voltage of +3.3 V and a control circuit coupled to the inverter, receiving output voltages of +12 V and +5 V and generating a control signal (not shown). In a desktop computer (as shown in FIG. 1), the DC/DC converter stage usually employs output voltages of +12 V and +5 V coupled to each other via an inductor and the weighted feedback of the two output voltages to accomplish the regulation of the two output voltages. The regulation of output voltage of +3.3 V is accomplished through an independent feedback. This kind of structure has the advantage of having a lower cost.
The rated power of a workstation computer is relatively higher (usually larger than 500 W), therefore, the output power of the +12 V output terminal of which is relatively higher too. If the structure of FIG. 1 is employed, it will cause two problems: 1. Due to the influence of the parasitic parameters of the coupled inductor between the outputs of +12 V and +5 V, the weighted feedback of two output voltages of +12 V and +5 V will interact each other and the output voltages can't be regulated stably, this is the cross regulation problem; 2. A single transformer bearing the whole output power raises the difficulty of heat dissipation design.
Thus, the switched-mode power supply in the workstation computer usually will employ a structure as shown in FIG. 2. FIG. 2 shows a circuit diagram of a DC/DC converter stage of a workstation computer in the prior art. In FIG. 2, the DC/DC converter stage 2 includes a first inverter, a first transformer T1 having a first primary winding N1p coupled to the first inverter, a first secondary winding N1s1 and a second secondary winding N1s2, a first rectifier coupled to the first secondary winding N1s1 and generating an output voltage of +5 V, a post regulator coupled to a node of the first secondary winding N1s1 and the second secondary winding N1s2, and generating an output voltage of +3.3 V, a first control circuit coupled to the first inverter, receiving the output voltage of +5 V and generating a first control signal (not shown), a second inverter, a second transformer T2 having a second primary winding N2p coupled to the second inverter and a third secondary winding N2s, a second rectifier coupled to the third secondary winding N2s and generating an output voltage of +12 V and a second control circuit coupled to the second inverter, receiving the output voltage of +12 V and generating a second control signal (not shown). The output voltage of +12 V accomplishes the closed-loop regulation of output voltage via the independent transformer T2 and the second rectifier. The output voltages of +5 V and +3.3 V commonly employ the same transformer T1, the output voltage of +5 V accomplishes the regulation of output voltage via the closed-loop feedback, and the output voltage of +3.3 V accomplishes the regulation of output voltage via another closed-loop feedback. Since the three output voltages are regulated through three respectively independent closed-loop feedbacks, therefore, there is not any cross regulation problem. Employing two transformers could make heat-dissipation design easier. Due to the different output voltages of the two transformers, the two transformers are independent from each other, and both are always working in the whole load range, thus the efficiency of the DC/DC converter stage in light load is not very high.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a large power multi-outputs power supply structure having a relatively high efficiency in a load range and a controlling method thereof.